Several trends presently exist in the semiconductor and electronics industry. Compared to prior generations of devices, modern devices are smaller and operate at lower power and higher frequencies. One reason for these trends is that personal devices are being fabricated to be relatively small and portable, thereby relying on a battery as their primary power supply. For example, cellular phones, personal computing devices, and personal sound systems are devices that are in great demand in the consumer market. In addition to being smaller and more portable, personal devices are also requiring increased memory and more computational power and speed. In light of these trends, there is an ever increasing demand in the industry for smaller and faster transistors used to provide the core functionality of the integrated circuits (ICs) used in these devices. The main reason to reduce circuit area is cost. Smaller die size implies more dies on a wafer. Consequently, if everything else remains the same, the cost per die is lower.
It can be appreciated that integrated circuits with more devices (i.e., having higher densities) tend to provide more functionality than integrated circuits with fewer devices (i.e., having lower densities). Accordingly, in the semiconductor industry there is a continuing trend toward manufacturing integrated circuits with higher densities. To achieve high densities, there are on-going efforts toward scaling down dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high densities, smaller feature sizes, smaller separations between features, and more precise feature shapes are required in integrated circuits fabricated on small rectangular portions of the wafer, commonly known as dies.
Generally speaking, integrated circuit designers organize transistors into standard cell libraries, where each standard cell within the library corresponds to a particular functional block. For example, one functional block could correspond to NOR functionality and another functional block could correspond to NAND functionality. These functional blocks provide designers with building blocks that they can tile together to achieve complex functionality. By re-engineering how features are arranged in standard cell libraries, higher densities can be achieved.